1. Field of the Invention
The invention relates generally to heat exchangers and more particularly to systems and methods for reducing leakage in heat exchangers which can allow fluids to pass between chambers in the heat exchangers and thereby contaminate either the heating/cooling system or the fluid to be heated/cooled.
2. Related Art
The use of heat exchangers to either heat or cool fluids is well known. There are many different types of heat exchangers. Many of these heat exchangers operate by passing fluids of different temperatures on opposite sides of a wall or membrane, so that heat energy from the hotter of the two fluids passes through the wall and into the cooler of the two fluids. For example, one fluid may be passed through a series of tubes that extend through a chamber containing the other fluid. As the fluid passes through the tubes, heat is exchanged through the walls of the tubes between the fluids.
One such type of heat exchanger is a U-tube heat exchanger. An exemplary design is illustrated in FIG. 1. In this figure, a first fluid is circulated into chamber 110, through U-shaped tubes 120 and into chamber 130. A second fluid is circulated in chamber 180 within shell enclosure 140. A tube sheet 150 separates chambers 110 and 130 from the interior of shell enclosure 140. Tube sheet 150 is connected to shell enclosure 140 with a gasket between them in order to create a seal between chambers 110/130 and shell enclosure 140. Each of tubes 120 passes through tube sheet 150 and is welded to the tube sheet to create a fluid passage from chamber 110 to chamber 130. Diaphragm 190 is used to seal chambers 110 and 130, and closure 160 is connected to shell enclosure 140 to provide support to diaphragm 190.
Because the heat exchanger may be used to process hazardous fluids, it is desirable to prevent leakage from chambers 110 and 130. It is also desirable to prevent cross-contamination from fluids passing between chamber 180 and chambers 110 and 130. It is therefore necessary to provide seals between tube sheet 150 and shell enclosure 140, as well as between the shell enclosure and diaphragm 190/closure 160. Conventionally, the seal between tube sheet 150 and shell enclosure 140 is provided by placing a simple gasket between opposing faces of tube sheet 150 and shell enclosure 140. This is shown in FIG. 1B. The seal between shell enclosure 140 and diaphragm 190 is conventional provided by welding the diaphragm to the shell enclosure.
Heat exchangers of the type illustrated in FIG. 1 are used in many applications. For example, this type of heat exchanger may be used in oil refineries for the purpose of cooling crude oil. These heat exchangers are, of course, very large and very expensive. Consequently, when it is necessary to repair one of these heat exchangers, the cost can be enormous, both in terms of the direct cost to repair the heat exchanger and in terms of the cost associated with downtime in the refinery. Because of the cost associated with problems in these heat exchangers, it is very important to minimize these problems to the greatest extent possible.
One of the problems that exists in the conventional heat exchanger design of FIG. 1 is that the temperature of the fluid in chambers 110 and 130 may change repeatedly, causing the materials forming the chambers to repeatedly expand and contract. Over time, this weakens the weld holding diaphragm 190 to shell enclosure 140, and may cause the weld to fail, allowing the fluid in chambers 110 and 130 (which are typically at a high pressure) to leak out of the heat exchanger.
Another problem is that leaks may develop in the seal between chambers 110/130 and chamber 180. This problem can be aggravated by the fact that the conventional gasket has a “blind” seal configuration. In other words, the gasket is positioned between two surfaces where it cannot be kept in position by a worker while the unit is being assembled—the worker is blind to the position of the gasket. As a result, the gasket often becomes pinched or twisted during assembly, so the unit must be disassembled and reassembled with a new gasket. Even when the gasket is properly installed, it is expected that the seal will need to be repaired/remanufactured every two to three years.
Leaks in this type of heat exchanger seal can be a very serious problem. For instance, when this type of heat exchanger is used to cool crude oil, leaks in the seal between the tube sheet and shell enclosure may allow crude oil to contaminate the cooling fluid, which may in turn foul other components of the cooling system. If this occurs, the repairs that are required may become even more extensive than simply replacing the gasket between the tube sheet and the shell enclosure. Even if the cooling system is not damaged, the cost of simply repairing the heat exchanger may easily be in the range of $500,000 to $800,000. While this amount may at first appear to be exorbitantly high, it should be noted that the repair is no simple task and includes: costs associated with shutting down the heat exchanger unit; the cost of the use of a crane which is necessary for assembly and disassembly of the unit; replacement of sealing surfaces (e.g., grinding down or un-welding stainless steel covers); heat treating repaired/remanufactured components; purging the heat exchanger; hydrogen bake-out; materials; labor; etc.